## *Confusion Regarding Cv and Cp* [ENDORSED]

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

Danah Albaaj 1I
Posts: 50
Joined: Fri Sep 29, 2017 7:07 am

### *Confusion Regarding Cv and Cp*

After doing multiple problems with entropy calculations with other classmates, UAs and TAs, as well as checking in the textbook and on here I am still confused on when to use Cv or Cp for ∆S= -nClnT2/T1. The problems all involved a diatomic gas at a changing volume and temperature, which meant I was supposed to use the Cp, however the answers were obtained using the Cv. I have heard many different responses on when to use which and am just hoping for some clarity on why this is happening in the problems.

Thank you!

Chem_Mod
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Joined: Thu Aug 04, 2011 1:53 pm
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### Re: *Confusion Regarding Cv and Cp*  [ENDORSED]

Since entropy is a state function we can separate the steps and then add them.

The overall process can be separated into two steps to make the individual calculations easier:

1: Decreased the volume, keep temperature constant.
2: Increase the temperature, keep volume constant.

See my detailed comments at viewtopic.php?f=133&t=26502&p=80676 which also has helpful student discussion.